For many years, antifreeze/heat transfer fluid concentrates have been used to form aqueous solutions used to cool internal combustion engines. These concentrates have also been used for deicing solutions used, for example, to device airplanes or power lines. Diols, polyhydric alcohols having two hydroxyl groups such as, for example, alkylene glycols, are often used as the base material for these antifreeze/heat transfer fluid concentrates. Diols typically make up 95% by weight of the antifreeze/heat transfer fluid concentrate and, after blending with water, about 40% to 60% by volume of the solution used for cooling the engine in a vehicle. Conventional antifreeze/heat transfer fluid concentrates have for years been formulated using ethylene glycol (EG) as the base material. EG has proved to be an efficient and cost effective means of providing freezing and boiling protection for engine coolants. In addition to its use in engine coolants, EG is used in a variety of other applications, including production of polyethylene terephthalate for use in polyester films, fibers, and resins.
EG has a number of properties that make it particularly suitable as an antifreeze in automobile engine coolants. When EG is added to water, the freezing point of the mixture is reduced to a safe level for cold weather. For example, a mixture of 50% water and 50% EG has a freezing point of 35.6° below zero Celsius (96° below zero Fahrenheit). In addition, EG has a very low vapor pressure compared to water. As a result, when a mixture of EG and water is heated, as in an internal combustion engine, the EG evaporates from the mixture at a rate very much less than the water. Accordingly, the mixture continues to have sufficient EG to prevent freezing in cold temperatures. Because of the relatively low vapor pressure of EG, mixtures of EG and water can retain their antifreeze characteristics for an extended period of time, while mixtures of water and more volatile alcohols cannot. The extended life of EG/water mixtures is particularly desirable in automobile engine coolants.
Another property of EG that is useful in an antifreeze is its specific gravity. EG has a specific gravity that is significantly greater than the specific gravity of water, and mixtures of EG and water have a higher specific gravity than pure water. For example, at 37.8° C. (100° F.), a mixture of 50% EG and 50% water has a specific gravity that is 6.2 percent greater than water at the same temperature. The concentration of EG in a mixture of EG and water can be easily determined by measuring the specific gravity of the mixture with a hydrometer, an inexpensive and easy to use device. Because the specific gravity is directly related to the concentration of EG in the solution, and the concentration of EG is in turn directly related to the freezing point of the solution, the specific gravity measurement can be used to determine easily whether there is sufficient EG in the solution.
While EG has served effectively as a freeze point depressant and boiling point elevator for engine coolants, its major disadvantage is its toxicity to humans and other mammals if ingested. In the late 1960's and early 1970's, toxicity and environmental concerns resulted in the elimination of chromate and arsenite additives from engine antifreezes and coolants. Since that time, however, formulations have changed little. Our continuing attention to environmental problems has caused renewed concern about the health effects and disposal problems associated with engine antifreezes/heat transfer fluid concentrates.
Reports and studies made by The American Association of Poison Control Center's National Data Collection System stated that there were over 1.1 million poisonings reported by 63 poison control centers. These 63 centers serve about half of the U.S. population. About 92% of the reported poisonings occurred in the home and the majority were accidental (89%). Children under six years of age were involved in 62% of the incidences and ingestion accounted for 77% of the poisoning exposures. This same report noted 2451 poisonings related to glycols with 2372 exposures being accidental and, of those, 765 were related to children under six years of age.
In considering toxicity and disposal issues associated with antifreeze/heat transfer fluid concentrates, it is helpful to break down an engine antifreeze/coolant into its component parts (similar parts are found in all EG and water-based thermal fluids):
1) Water—the primary heat removal fluid. The water content of a solution used as an engine coolant is typically 40% to 70% by volume depending upon the severity of the winter climate. In some warm weather areas, freezing temperatures are not encountered, and water with a corrosion prevention additive is used, or EG (with additives) is added solely to raise the boiling point of the coolant solution.
2) Freezing Point Depressant and Boiling Point Elevator—in most cases EG is used in a range of 30% to 60% by volume to prevent freezing of the water during the winter. Addition of EG also raises the boiling point of the solution, and the same range of EG is typically used during the summer in temperate regions and year round in warmer climates.
3) Additive Package—typically contains several different chemicals that are initially added to the glycol to form an antifreeze or concentrate and eventually blended with water to form the coolant. These additives are designed to prevent corrosion, deposit formation, and foaming, and are typically each present in concentrations of 0.1% to 3% by weight of the coolant concentrate.
4) Contaminants—build up as the engine is used, and result from the following:                thermal or oxidative breakdown of glycol        lube oil and fuel accumulation        metals from cooling system corrosion        
LD50 values (acute oral toxicity ratings) are useful in comparing the relative toxicities of substances. The LD50 value for a substance is the dose level (in mg/kg of body weight) administered at the beginning of a two-week period required to kill 50 percent of a group of laboratory rats. A coolant material that has an LD50 value of 5,000 mg/kg or lower may be classified as hazardous, with lower LD50 ratings indicative of increased toxicity. EG has an acute oral toxicity (LD50) of 4,700 mg/kg. Although marginally hazardous by this rating system, EG is a known toxin to humans at relatively low levels (reported as low as 1,570 mg/kg in Toxic Release Inventory Reporting, Notice of Receipt of Petition, Federal Register, Vol. 63, No. 27, Feb. 10, 1998) and consequently is classified by many regulatory authorities as a hazardous material. In addition, EG has a sweet smell and taste, making it attractive to children and animals.
The toxicity associated with EG is caused by the metabolites of EG, some of which are toxic. EG, when ingested, is metabolized to glycoaldehyde by alcohol dehydrogenase (ADH), an enzyme necessary for the conversion. Glycoaldehyde further metabolizes to glycolic acid (glycolate). The accumulation of glycolic acid causes metabolic acidosis. Also, glycolic acid accumulation correlates with a decrease in arterial bicarbonate concentration. Some of the glycolic acid metabolizes to glyoxylic acid (glyoxylate), which further metabolizes to oxalic acid (oxylate). Oxalic acid binds to serum calcium in the bloodstream, and precipitates as crystals of calcium oxalate.
Characteristic symptoms observed with EG ingestion include anion gap metabolic acidosis, hypocalcemia, cardiac failure, and acute oliguric renal failure. Calcium oxylate crystals in many cases can be found throughout the body. Calcium oxylate crystals in the kidneys cause or are associated with the development of acute renal failure.
There are known to be two basic treatments for EG poisoning, both interfering with action of the ADH enzyme to prevent the first metabolism in the chain of events, namely, the metabolism of EG into glycoaldehyde. Until recently, ethanol had been the standard antidote for EG poisoning. Currently, there is only one FDA-approved antidote: fomepizole (4-methylpyrazole), which is trademarked Antizol and was approved by the FDA in 1997. As reported by Jacobsen in “New treatment for ethylene glycol poisoning”, New Eng. J. of Med., Vol. 340, No. 11, Mar. 18, 1999, the series of required fomepizole treatments costs approximately $4,000. Due to the relatively high cost of fomepizole treatment, ethanol is still often used to treat EG poisoning.
Ethanol is the substrate for the ADH enzyme, which means that the ADH enzyme has a great affinity for ethanol to the exclusion of other substances. If enough ethanol is present, the ingested EG gets “crowded out” by the ethanol and is prevented from becoming metabolized. Ethanol, rather than EG, gets metabolized and the metabolites of ethanol are generally less harmful. While the ethanol is being metabolized, the unmetabolized EG has time to pass through the body and be expelled in wastes.
The amount of ethanol required to treat EG poisoning is considerable. As reported by Stipetic and Hobbs, “Tex Tox: Shaken, Not Stirred”, Central Texas Poison Center, Jan. 8, 1999, for maximum efficacy, the desired serum ethanol concentration should be maintained between 100-150 mg/dL. This concentration should be maintained until levels of EG are undetectable and the metabolic acidosis has been corrected. Patients that are treated with ethanol (treatment that may last several days) become intoxicated and are at risk for developing hypoglycemia. Additionally, patients must be monitored for elevated liver enzymes.
Fomepizole is a far more effective treatment for EG poisoning than is ethanol because it blocks the action of the ADH enzyme so as to prevent the conversion of EG into glycoaldehyde. Far less fomepizole is required for treatment than ethanol. For example, a fomepizole treatment regime may consist of administering a loading dose of 15 mg/kg, followed by doses of 10 mg/kg every 12 hours for 4 doses, then 15 mg/kg every 12 hours thereafter until EG levels have been reduced below 20 mg/dL. A victim of EG poisoning, whether treated with ethanol or fomepizole, also requires treatment with sodium bicarbonate to counteract ongoing production of organic acids and hemodialysis to remove the glycolic acid that may have been produced by metabolism of EG between the time of ingestion and the start of treatment.
Ethanol is not a practical ADH enzyme inhibitor for use in heat transfer fluid concentrates because it is relatively ineffective for this purpose. Also, ethanol, with a boiling point of 169° F. (76° C.), is too volatile for a coolant ingredient. Additionally, its flash point of 65° F. (18.3° C.) is unacceptable. Fomepizole lacks practicality as an ADH enzyme inhibitor for use in an antifreeze/heat transfer fluid concentrate because of its great expense. Thus, the two heretofore known substances for the treatment of EG poisoning are poor candidates for use as preventatives of poisoning in mixtures with EG. The inventors have discovered that some substances which can be practically incorporated into EG-based heat transfer fluid concentrates can act as ADH enzyme inhibitors.
One accepted means of estimating the toxicity of a formulation containing hazardous ingredients is a calculation method. As described in the World Health Organization Classification of Pesticides by Hazard and Guidelines to Classification 1998-99, the LD50 of a mixture containing substances having known LD50 values can be estimated by the following formula:CA/TA+CB/TB+ . . . +CZ/TZ=100/TMxtr Where:
C=the % concentration of constituents A, B . . . , Z in the mixture.
T=the acute oral (rat) LD50 values of the constituents A, B . . . , Z.
TMxtr=the estimated acute oral (rat) LD50 value of the mixture.
The calculation method described above is used in Table 1 to compare the sources of toxicity in the standard ASTM antifreeze/coolant formulation, GM-6038, which is a typical EG-based antifreeze concentrate.
TABLE 1LD50 EstimateCTINGREDIENTWT PERCENTAGELD50 (mg/kg)C/T (×105)EG95.6547002035NaNO30.2037505Na2B4O7-5H2O1.00266038Na2SIO3-5H2O0.15128012Na3PO4-12H2O0.45170003NaMBT(50%0.55312018SOLN)NaOH0.2050040PLURONIC L-610.05—0GREEN DYE0.005—0WATER1.75—0Sum of Factors2151EstLD50 = Reciprocal of Sum of Factors × 100:4649
As shown in Table 1, EG is the largest single component in the formulation, and its LD50 value largely determines the estimated formulation LD50. Because they are present in very low concentrations, the small contribution of the Pluronic L-61 and the dye were not considered. Also, the water present to solubilize the additives will tend to dilute the toxic effects of the other components and raise the LD50 level of the formulation. The water is assumed to add no toxicity.
Additive packages may be added to an antifreeze/heat transfer fluid concentrate to replenish inhibitors. Supplemental coolant additives (SCAs) used to replenish inhibitors will often consist of from 5 to 15 different chemicals. These additives, as shown below, are broken down into major and minor categories depending on the amount used in an engine antifreeze/heat transfer fluid formulation:
MAJOR (0.05 TO 3%)MINOR (0.05%)BUFFERDEFOAMERCORROSION INHIBITORSDYESCALE INHIBITORSURFACTANTCHELANTS
The materials typically used as minor additives should not be of significance to the toxicity of engine antifreeze/heat transfer fluids because these materials usually have a relatively low toxicity and they are present in small quantities. Nitrite has the highest toxicity rating of the additives still frequently used in engine coolants, with an LD50 for rats of 85 mg/kg (in the range of arsenite). The triazoles are moderately toxic while most of the other materials typically used in SCAs have LD50 values that are in the same range as table salt and aspirin.
The toxicity of some antifreeze/heat transfer fluid additives is affected by their alkalinity. The more alkaline forms of silicate, phosphate, and borate have lower LD50 values, and accordingly have a higher toxicity rating. Thus, the more alkaline metaborate (Na2B2O4.4H2O) has an LD50 value of 1,700 mg/kg compared to the less alkaline tetraborate with a value of 2,300 mg/kg to 3,300 mg/kg. Similarly, sodium silicate with an SiO2/Na2O ratio of 1 has an LD50 value of 600 mg/kg as compared to 1,600 mg/kg for the less alkalineh silicate with an SiO2/Na2O ratio of 2.
The toxicity, or more appropriately the skin corrosiveness, of metasilicate (pH 13 @ 5%) is greatly neutralized when blended into an antifreeze/heat transfer fluid with a pH in the range of 10. The best example of this is the blending of phosphoric acid with potassium hydroxide in an antifreeze coolant or liquid SCA. The end product is a mildly alkaline salt that exhibits much less toxicity and corrosiveness than the starting materials.
The chemicals that may be included in an antifreeze/heat transfer fluid additive package have many common uses. Some of these chemicals, such as adipate, benzoate, carbonate, nitrite, phosphate, and silicate, are used in foods. Even nitrite, which has the lowest LD50 (i.e., the highest oral toxicity) of any of the common additives, is used in very small quantities as a food preservative and in medicines. Borate, benzo triazole, carbonate, phosphate, silicate, and triethanolamine are used in soaps and detergents. As with all chemical products, additive chemicals should be handled with care, but in a formulated engine antifreeze/heat transfer fluid, these chemicals present no extraordinary health risk.
Worldwide nearly 400 million gallons of antifreeze/heat transfer fluid concentrates are sold every year. It is estimated that a significant percentage of this volume is disposed of improperly, resulting in contamination of the environment. Improper disposal by consumers is a major cause of this environmental contamination. Another major source of environmental contamination is leakage, spills and overflows from heavy duty vehicles. Experience with heavy duty vehicles shows that it is common to lose 10% of the antifreeze/heat transfer fluid volume after every 12,000 to 18,000 miles of operation due to leaks in the system components, such as the water pump, hose or clamps or radiator core. This rate of loss is equal to about one gallon/month for the typical highway truck, which is the equivalent of a leakage rate of one drop per minute. An antifreeze/heat transfer fluid leak rate of one drop per minute is likely to go unnoticed, but can in total add up to a significant loss.
In some operations using heavy duty vehicles, overflows account for far more antifreeze/heat transfer fluid loss than low level leaks at the water pump, hose clamps or radiator core. Overflows occur due to overheating or when a cooling system is overfilled. When a cooling system is overfilled, operation of the engine heats the antifreeze/heat transfer fluid, causing expansion of the fluid that cannot be contained in the system. Pressure relief valve lines typically allow excess fluid to escape to the ground. Small EG spills and leaks (less than a gallon) of antifreeze/heat transfer fluid eventually will biodegrade with little impact to the environment. However, before biodegradation occurs, these spills and leaks can present a toxic danger to pets and wildlife.
The environmental concerns detailed above, particularly as related to spillage and oral toxicity, are related to antifreeze/heat transfer fluid concentrates in which the major fraction (about 95%) is EG (EG). EG is most hazardous when it exists in concentrates, e.g., as sold to consumers as antifreeze in chain stores and markets or when stored in commercial businesses (i.e., 55 gal drums).
The use of EG mixed with water in an engine coolant solution can also result in release of concentrated EG into the environment. At 200° F. (93.3° C.), the vapor pressure of water is 600 mm Hg, while the vapor pressure of EG at that temperature is just 10 mm Hg. Antifreeze/heat transfer fluid solutions used in internal combustion engines will typically start as 50% antifreeze (95% of the antifreeze being EG) and 50% water. Due to the difference in vapor pressure between water and EG, the solution will tend to become more concentrated in EG as water evaporates through “breathing” of the cooling system. Also as a result of the vapor pressure difference, heated antifreeze/heat transfer fluid solution that has been expelled from a cooling system will readily concentrate toward straight EG in the environment, increasing its oral toxicity. The hotter the solution expelled from the cooling system, the more rapidly the water content will pass into the atmosphere, leaving the more concentrated EG behind. Even though temporarily reduced in its hazardous rating level when diluted with water, EG and water-based antifreeze/heat transfer fluid solutions will approach EG's concentrated LD50 value of 4,700 mg/kg when the solution is passed out of an automobile's cooling system vent into the environment. When the water is removed from the coolant solution, the antifreeze/heat transfer fluid concentrate is essentially returned to its initial concentrated state, and it is released into the environment as a hazardous, poisonous substance.
In recent years, a base fluid concentrate containing about 95% propylene glycol (PG) has been used as a substitute for EG in many antifreeze/heat transfer fluid concentrate formulations to avoid the toxicity associated with EG. PG has an LD50 value of 20,000 mg/kg as compared to EG's 4,700 mg/kg. PG is so non-toxic that it is approved by the U.S. Food and Drug Administration as a food additive. The greatest impediment to more widespread usage of PG as a base fluid for antifreeze/heat transfer fluid concentrates is its relatively high cost as compared to EG. Although PG has been used in some applications, EG remains the antifreeze base fluid of choice among the world's major antifreeze/heat transfer fluid concentrate manufacturers.
Accordingly, among the objects of the present invention is to provide an EG composition containing a substance, such as for example an ADH enzyme inhibitor, that reduces the toxicity of the EG composition while allowing the composition to retain the properties that make EG desirable for many commercial uses, such as an antifreeze and heat transfer fluid, and a deicer. Another object of the invention is to reduce the toxicity of bulk quantities of EG. Other objects of the invention will be apparent to those skilled in the art based upon the description set forth herein.